Cubic boron nitride (which hereinafter may be abbreviated as cBN) is second to diamond in hardness and has chemical stability higher than that of diamond. Thus, cubic boron nitride is increasingly employed as a grinding material, polishing material, or cutting material. A variety of methods for producing cBN have been proposed. Among them, the best known and the most widely employed in industry is a method in which hexagonal boron nitride (which hereinafter may be abbreviated as hBN) is held in the presence of a catalyst substance (which may be referred to as a solvent) under conditions in which cBN remains thermodynamically stable (approximately 4.0 to 6.0 GPa, approximately 1,400 to 1,600° C.), to thereby cause hBN to undergo a phase transition to form cBN.
There have been already proposed catalyst substances composed of an alkali metal, an alkaline earth metal, an alkali metal nitride, an alkali metal boronitride, an alkaline earth metal nitride, an alkaline earth metal boronitride, or the like.
For example, lithium nitride (Li3N) and lithium boronitride (Li3BN2) have been proposed as the catalyst substance (see Patent Document 1). However, cubic boron nitride produced in the presence of lithium nitride or lithium boronitride is generally in the form of finely divided particles having a particle size of 50 μm or less and insufficiently developed crystal planes. Thus, such cubic boron nitride exhibits poor performance as abrasive grains.
Lithium calcium boronitride (LiCaBN2) has also been proposed as a catalyst substance (see Patent Document 2). The cBN produced in the presence of the catalyst substance assumes a generally spherical form and has excellent mechanical strength.
A mixture of LiMBN2 (M represents an alkaline earth metal) and Li8SiN4 and a mixture of LiMBN2 and Ca5Si2N6 have also been proposed as a catalyst substance (see Patent Documents 3 and 4). The cBN produced in the presence of any of these catalyst substances has a developed (111) crystal plane and excellent mechanical strength.
There has been proposed a method for producing cubic boron nitride employing a carbon source, an Si source, an alkali metal hydride, an alkaline earth metal hydride, or another catalyst substance (see Patent Document 5). The cBN produced in the presence of any of these catalyst substances assumes the form of dense, transparent crystalline particles having a sharp edge.
There has also been proposed, as a catalyst for synthesizing cubic boron nitride, a mixture of at least one of lithium nitride and lithium boronitride with at least one of metallic magnesium and magnesium boride, wherein magnesium atoms are contained in an amount of 4 to 85 parts by mole and lithium atoms are contained in an amount of 100 parts by mole (see Patent Document 6). When cBN is synthesized in the presence of the catalyst, the proportion of the crystal grains having a grain size larger than 100 μm increases, crystal planes are more developed, and phase transition ratio from hBN to cBN increases, as compared with the case in which lithium nitride or lithium boronitride is used singly.
(Patent Document 1)
Specification of U.S. Pat. No. 3,772,428
(Patent Document 2)
Japanese Examined Patent Application, Second Publication No. S61-283
(Patent Document 3)
Japanese Examined Patent Application, Second Publication No. H05-94
(Patent Document 4)
Japanese Examined Patent Application, Second Publication No. H05-95
(Patent Document 5)
Japanese Examined Patent Application, Second Publication No. H04-2296, and Specification of U.S. Pat. No. 5,000,760
(Patent Document 6)
Japanese Examined Patent Application, Second Publication No. S53-047239
When the atomic ratio of magnesium with respect to lithium is increased, phase transition ratio into cBN increases; however, when more than 85 parts magnesium atoms by mole are mixed with 100 parts of lithium atoms by mole in the synthesizing catalyst, the number of defects such as holes or the like in the crystal planes of the cBN grains increases; therefore, the atomic ratio of magnesium cannot be increased to more than 85 parts although improvement in phase transition ratio is expected.
An object of the present invention is to solve the aforementioned problems, i.e., the problems in that the number of defects such as holes or the like in the crystal planes of the produced cBN grains increases when the atomic ratio of magnesium with respect to lithium is increased, and thereby to enable use of more than 85 parts magnesium atoms by mole with respect to 100 parts of lithium atoms by mole in the synthesizing catalyst so as to provide a method by which the phase transition ratio into cBN increases, the number of defects such as holes or the like in the crystal planes of the produced cBN grains is decreased, and performance of abrasive grains is improved. Further objects of the present invention are to provide cBN produced through the above method, to provide a grinding wheel of such cBN, and to provide a sintered cBN compact.